Path computation based on dynamic performance monitoring systems and methods in optical networks

ABSTRACT

The present disclosure provides dynamic performance monitoring systems and methods for optical networks to ascertain optical network health in a flexible and accurate manner. The present invention introduces accurate estimations for optical channel performance characteristics based either on existing channels or with a dynamic optical probe configured to measure characteristics on unequipped wavelengths. Advantageously, the dynamic performance monitoring systems and methods introduce the ability to determine physical layer viability in addition to logical layer viability.

CROSS-REFERENCE TO RELATED APPLICATION

The present non-provisional patent application/patent is a continuationof U.S. patent application Ser. No. 13/872,550, filed on Apr. 29, 2013,and entitled “COHERENT PROBE AND OPTICAL SERVICE CAHNNEL SYSTEMS ANDMETHODS FOR OPTICAL NETWORKS,” which is a continuation-in-part of U.S.patent application Ser. No. 12/623,711, filed on Nov. 23, 2009, andentitled “DYNAMIC PERFORMANCE MONITORING SYSTEMS AND METHODS FOR OPTICALNETWORKS” which claims the benefit of priority of U.S. ProvisionalPatent Application No. 61/120,649, filed on Dec. 8, 2008, and entitled“DYNAMIC PERFORMANCE MONITORING SYSTEMS AND METHODS FOR OPTICALNETWORKS,” the contents of each are incorporated in full by referenceherein.

FIELD OF THE INVENTION

The present invention relates generally to optical networks and systemsand methods for monitoring and assessing the performance of the same.More particularly, the present invention provides dynamic performancemonitoring systems and methods for optical networks that allow opticalnetwork health to be ascertained in a flexible and accurate manner. Theconfiguration and performance of the optical networks may then beadjusted accordingly to achieve desired characteristics and/orperformance goals. Thus, the present invention provides valuablediagnostic tools.

BACKGROUND OF THE INVENTION

Optical (i.e., transport) networks and the like (e.g., wavelengthdivision multiplexing (WDM), Synchronous Optical Network (SONET),Synchronous Digital Hierarchy (SDH), Optical Transport Network (OTN),Ethernet, and the like) at various layers are deploying control planesystems and methods. Control plane systems and methods provide automaticallocation of network resources in an end-to-end manner. Exemplarycontrol planes may include Automatically Switched Optical Network (ASON)as defined in G.8080/Y.1304, Architecture for the automatically switchedoptical network (ASON) (February/2005), the contents of which are hereinincorporated by reference; Generalized Multi-Protocol Label Switching(GMPLS) Architecture as defined in Request for Comments (RFC): 3945(October/2004) and the like, the contents of which are hereinincorporated by reference; Optical Signaling and Routing Protocol (OSRP)from Ciena Corporation which is an optical signaling and routingprotocol similar to PNNI (Private Network-to-Network Interface) andMPLS; or any other type control plane for controlling network elementsat multiple layers, and establishing connections there between.

The GMPLS, ASON, etc. standards are driving increasing levels of dynamicoptical network reconfigurability. Optical signal propagation is aninherently analog process, and monitoring analog network performance iscritical to dynamic reconfigurability. Both optical network design andreconfiguration require the use of optical path computation softwarethat computes expected signal performance based on specific networkphysical characteristics. An example of such path computation softwareis the Path Computation Element (PCE) currently under consideration inthe IETF. These must still be validated against field measurements, asthere are large uncertainties in the optical fiber and installedequipment as well as possible aging errors. Networks may havewavelengths with several technology generations supporting a variety ofdata rates, modulation formats, and the like.

The current state of the art in deployed networks is limited to threetypes of measurements. First, existing channels provide a measure ofboth pre-corrected and post-corrected Forward Error Correction (FEC)error counts. These are only available for specific lightpaths, wherechannels with embedded FEC are already installed and operational.Further, pre-FEC bit error rate (BER) is only accurate at high values.At lower values of BER, the counts only provide an upper boundmeasurement due to the presence of dynamic control algorithms, whichstop working once a specific bound is reached. Finally, no informationis provided that can be used to predict the performance of channels witha different bit rate and modulation format.

Second, channel power levels are available at various points in thesystem, either as an aggregate total or for individual channels as atOptical Channel Monitor (OCM) points. These provide some indication ofthe overall system health, but can say very little about specificchannel performance or about path suitability for additional channels.Third, some recent monitors have added Optical Signal-to-Noise Ratio(OSNR) measurement capability, which provides an indication of one ofmore major optical signal impairment mechanisms.

While some signal quality measurement approaches exist, they do notprovide sufficient information to accurately estimate new channelperformance, or to validate the accuracy of the path computationcalculation. What is missing is ability to extract the following:

-   -   More accurate OSNR measurement;    -   Estimation for residual Chromatic Dispersion;    -   Estimation for Polarization Dependent Loss;    -   Estimation for Polarization Mode Dispersion;    -   Estimation for inter-channel nonlinear effects, such as        Cross-Phase Modulation (XPM) and Four Wave Mixing (FWM);    -   Estimation for intra-channel nonlinear effects, such as        Self-Phase Modulation (SPM), iXPM, iFWM; and    -   Estimation for possible bandwidth narrowing due to in-line        optical filtering (for example, Optical Add-Drop Module (OADM)        filters).

With respect to Optical Service Channels (OSCs), conventional OSCs usefixed wavelength intensity modulated direct detection (IMDD)transponders in an out-of-band wavelength. Feedback on performance datain conventional systems use coherent transponders to glean some linkperformance information on a path-by-path basis; however, the majorityof link characterization is done before the system gets deployed.Conventional optical service channels offer only limited linecharacterization with the main purpose providing communication betweennetwork components.

As described herein, link characterization in conventional systemsincludes major cost and complexity in designing and deploying opticalnetworks. The process of producing link budgets (optical performance)involves collecting data on the network, and propagation simulation.This is a lengthy, expensive, time consuming process. The mainlimitation of this process is that data is only gathered once in thelifetime of the system—before it is even deployed. Normally, since OSC'shave been traditionally IMDD receivers, there is the need to filter theoutput of each span so that only the OSC wavelength range is present onthe OSC receiver. Since this is the case, it is normally the practice touse an out of band wavelength which is not amplified by the EDFAs in thesystem. This saves bandwidth for data-bearing traffic, but limits theusefulness of the OSC measurements since it cannot measure the ASEgenerated by the amplifiers themselves. Therefore, conventional OSCsoffer some direct characterization which is limited to loss, latency andwith a coarse precision, chromatic dispersion.

Other characteristics of the line system must be calculated indirectlyfrom these measurements and the one-time, start-of-life fibercharacterization data using detailed knowledge of the system design, forexample, the noise figure of the amplifiers, user knowledge of the fibertype, etc. Active feedback on performance data in current systems hasbeen suggested using coherent transponders to glean some linkperformance information. However, this information is available only forin-service light paths, making it difficult or impossible to locate thesource of any particular effect or degradation within those paths, andresults in a complete lack of information on paths which are notcurrently in use.

BRIEF SUMMARY OF THE INVENTION

In an exemplary embodiment, an optical system includes a coherentoptical transmitter; a coherent optical receiver; and a digitalprocessing block connected to the coherent optical transmitter and thecoherent optical receiver, wherein the digital processing blockselectively operates the coherent optical transmitter and the coherentoptical receiver as one of an optical probe and an optical servicechannel. The coherent optical receiver can be configured to tune to oneof a plurality of wavelengths, and, when operating as the optical probe,the digital processing block determines optical channel performancecharacteristics of the one of the plurality of wavelengths. The opticalchannel performance characteristics can include any of OSNR measurement,residual Chromatic Dispersion, Polarization Dependent Loss, PolarizationMode Dispersion, inter-channel nonlinear effects, intra-channelnonlinear effects, cross-phase modulation, and bandwidth narrowing. Thedigital processing block can provide the optical channel performancecharacteristics to an optical control plane for inclusion in an opticalpath computation function associated with the optical control plane. Thecoherent optical transmitter can be configured to tune to an opticalservice channel wavelength outside an amplification band, and, whenoperating as the optical service channel, the digital processing blockinterfaces with the coherent optical transmitter and the coherentoptical receive for data transmission.

The data transmission can utilize a low data rate and robust modulationformat such as a dual-polarization binary phase-shift keying modulationformat. The optical system can further include a first optical deviceconnected to the coherent optical receiver and a downstream opticalfiber, wherein the first optical coupler is configured to provide anupstream signal to the coherent optical receiver; and a second opticaldevice connected to the coherent optical transmitter and an upstreamoptical fiber, wherein the second optical coupler is configured toprovide a downstream signal from the coherent optical receiver. Thefirst optical device and the second optical device each can include afour port coupler including a common in port, a common out port, an OSCadd port, and an OSC bypass port. The coherent optical transmitter andthe coherent optical receiver can utilize oversampling of a heterodyneintermediate frequency (IF) with adaptive digital filtering.

In another exemplary embodiment, an optical network includes a pluralityof nodes interconnected through a plurality of links; a control planecommunicatively coupled to each of the plurality of nodes; and acoherent optical system between each of the plurality of nodesselectively operating as one of an optical probe and an optical servicechannel therebetween; wherein the control plane includes an optical pathcomputation function configured to provide estimation of optical channelperformance characteristics based on measurements by the optical probe.The coherent optical system can include a coherent optical transmitter;a coherent optical receiver; and a digital processing block connected tothe coherent optical transmitter and the coherent optical receiver,wherein the digital processing block selectively operates the coherentoptical transmitter and the coherent optical receiver as one of theoptical probe and the optical service channel. The coherent opticaltransmitter can be configured to tune to one of a plurality ofwavelengths, and, when operating as the optical probe, the digitalprocessing block determines optical channel performance characteristicsof the one of the plurality of wavelengths. The optical channelperformance characteristics can include any of OSNR measurement,residual Chromatic Dispersion, Polarization Dependent Loss, PolarizationMode Dispersion, inter-channel nonlinear effects, intra-channelnonlinear effects, cross-phase modulation, and bandwidth narrowing.

The digital processing block can provide the optical channel performancecharacteristics to an optical control plane for inclusion in an opticalpath computation function associated with the optical control plane. Thecoherent optical transmitter can be configured to tune to an opticalservice channel wavelength outside an amplification band, and, whenoperating as the optical service channel, the digital processing blockinterfaces with the coherent optical transmitter and the coherentoptical receive for data transmission. The data transmission can utilizea low data rate and robust modulation format such as a dual-polarizationbinary phase-shift keying modulation format. The coherent optical systemcan further include a first optical device connected to the coherentoptical receiver and a downstream optical fiber, wherein the firstoptical coupler is configured to provide an upstream signal to thecoherent optical receiver; and a second optical device connected to thecoherent optical transmitter and an upstream optical fiber, wherein thesecond optical coupler is configured to provide a downstream signal fromthe coherent optical receiver. The first optical device and the secondoptical device each can include a four port coupler including a commonin port, a common out port, an OSC add port, and an OSC bypass port. Thecoherent optical transmitter and the coherent optical receiver canutilize oversampling of a heterodyne intermediate frequency (IF) withadaptive digital filtering.

In yet another exemplary embodiment, a method includes operating acoherent optical probe between a first node and a second node; tuningthe coherent optical probe to one of a plurality of wavelengths;measuring optical channel performance characteristics of the one of theplurality of wavelengths utilizing a digital processing block at thesecond node; subsequent to the measuring, tuning the coherent opticalprobe to an optical service channel wavelength; and operating thecoherent optical probe as an optical service channel between the firstnode and the second node.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated and described herein with referenceto the various drawings, in which like reference numbers are used todenote like system components/method steps, as appropriate, and inwhich:

FIG. 1 is a schematic diagram and illustrates an optical networkincluding a plurality of interconnected nodes according to an exemplaryembodiment of the present invention;

FIG. 2 is a schematic diagram and illustrates different types ofsoftware modules associated with an optical control plane according toan exemplary embodiment of the present invention;

FIG. 3 is a flow diagram and illustrates a hybrid path computationcapability that includes a centralized (offline) network planning toolat the network management layer and distributed (online) pathcomputation engines at each network element for use by the control planeaccording to an exemplary embodiment of the present invention;

FIG. 4 is a schematic diagram and illustrates a hybrid path computationmechanism in an optical network according to an exemplary embodiment ofthe present invention;

FIG. 5 is a schematic diagram and illustrates a network managementsystem with a path pre-computation function according to an exemplaryembodiment of the present invention;

FIG. 6 is a schematic diagram and illustrates the optical network ofFIG. 1 with a plurality of optical probes included at trafficorigination/termination locations according to an exemplary embodimentof the present invention;

FIG. 7 is a schematic diagram and illustrates a dynamic probetransmitter for an optical probe according to an exemplary embodiment ofthe present invention;

FIG. 8 is a schematic diagram and illustrates a dynamic probe receiverfor an optical probe according to an exemplary embodiment of the presentinvention;

FIG. 9 is a schematic diagram and illustrates a block diagram of aserver configured to, responsive to computer-executable code, perform anoptical path computation function according to an exemplary embodimentof the present invention;

FIG. 10 is a schematic diagram and illustrates a coherent OSC systemaccording to an exemplary embodiment of the present invention; and

FIG. 11 is a schematic diagram and illustrates a four-port coupler forthe coherent OSC system of FIG. 10 according to an exemplary embodimentof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In various exemplary embodiments, the present invention provides dynamicperformance monitoring systems and methods for optical networks thatallow optical network health to be ascertained in a flexible andaccurate manner. The present invention introduces accurate estimationsfor optical channel performance characteristics based either on existingchannels or with a dynamic optical probe configured to measurecharacteristics on unequipped wavelengths. Advantageously, the dynamicperformance monitoring systems and methods introduce the ability todetermine physical layer viability in addition to logical layerviability. The present invention provides span-by-span orsection-by-section optical characterization of and nodal communicationswithin a terrestrial or submarine fiber optical transmission system overthe life of the system.

The present invention includes an optical probe element positioned atvarious points throughout the optical network. The optical probeincludes a transmitter and receiver portion. The transmitter portion ofthe probe produces an optical pulse train modulated with PRB S data, andwith a tunable wavelength and duty cycle and repetition rates selectableto represent signals of interest, i.e. 10 G, 20 G, 40 G, 100 G, etc.signals. The receiver portion of the probe uses asynchronous sampling toacquire signal diagrams. A Digital Signal Processor (DSP) analyzesacquired signals as a function of optical power, data rate, and tunedsampling timing, and provides a separable measurement of ASE, SPM,Filter narrowing, Chromatic Dispersion, and Polarization Mode dispersion(PMD) distortions. The measurements are cross-correlated to thepropagation computations, which account for exact characteristics ofdata carrying wavelengths.

The optical probes can be deployed at all places where traffic demandsoriginate/terminate or at a subset of such traffic endpoints. Underoperator or automated control, these optical probes could be exercisedto validate unused light paths across the network, in effect certifyingthese as usable by routing policies, either control plane based oroperator/manually instigated. This can be a powerful aid in assuringthat lightpaths can be established, or restored, with high confidence,particularly by control plane mechanisms.

Under automated control, the probes could be used, for example, duringoff-peak hours, to systematically validate the performance of potentiallightpaths between points in the network and to build a database oflightpaths supporting a desired quality of service for future pathprovisioning or restoration. Note that the probes can be used inconjunction with existing optical transceivers. The existing opticaltransceivers can also provide optical parameters on the various fiberlinks over which they are provisioned.

In another exemplary embodiment, the present invention includes anoptical service channel (OSC) including a low-cost coherent opticalmodule which is specifically designed for span-by-span opticalcharacterization in addition to the communications normally associatedwith OSCs. The coherent OSC can operate similar to the optical probeswith additional functionality as an OSC while concurrently providingsystem-level algorithms to extract measurements and importantperformance data from the system both while it is being installed andover the life of the system.

Advantageously, the present invention can provide a significantreduction in time to provide link budgets, reduction in cost and testtime for link budget model development, optical characterization overthe life of the network, and the like. With this information, opticalnetwork can suggest modem, optical performance, and maximum capacity.The owner of the optical network can purchase plug and play componentswithout the need for propagation simulation. The present inventionincludes advanced network performance monitoring, including PMD, CD,PDL, Latency; an inexpensive, low bit rate implementation; and supporton very difficult networks.

Referring to FIG. 1, an optical network 100 with a plurality ofinterconnected nodes 102 is illustrated according to an exemplaryembodiment of the present invention. Each of the interconnected nodes102 can include a wave division multiplexing (WDM) terminal, aReconfigurable OADM (ROADM), a Wavelength Selective Switch (WSS), anoptical switch, a router, a data switch, a SONET/SDH platform, or thelike. The interconnected nodes 102 are connected through a plurality ofconnections 104, which include optical fiber carrying one or morewavelengths between adjacent nodes 102.

The optical network 100 can further include a control plane 106. Forexample, the control plane can include Optical Signaling and RoutingProtocol (OSRP), Automatically Switched Optical Networks (ASON),Generalized Multi-Protocol Label Switching (G-MPLS), and the like. Thecontrol plane 106 introduces intelligence in an optical transportsystem. It can perform many functions such as automatic resourcediscovery, distributing network resource information, establishing andrestoring connections dynamically across the network, and the like.Accordingly, the control plane 106 is introducing layer two/threefunctionality in the optical network 100, i.e. the photonic layer, i.e.to communicate control plane information across a signalingcommunication network.

Of note, the role of Path Computation at the photonic layer is centralto the lightpath routing calculation required to set up either primaryor backup (protection/restoration) connections. Until recently, theWDM-based photonic layer has been used to provide static physical layerconnectivity in carriers' networks and has typically been exempt fromthe requirement to reconfigure and respond to changes in trafficpatterns. In carriers' networks, network reconfiguration is performed atthe higher layers that are clients to the optical layer and provisioningof the optical layer is usually accomplished by a manually intensiveprocess of segment-by-segment interconnection, requiring coordinationbetween the carrier's various network management, operations, andinventory systems.

With the need to cope with the uncertainty of fast-changing data traffic(and consequently reduce operational expenses), carriers have beenredesigning their operations support systems over recent years toautomate network management and speed up their provisioning processes.This has been further supported by the introduction of advancedsignaling and routing protocols that automate key administration andmanagement functions and help the carrier to establish and tear downthese connections in a rapid manner, i.e. the control plane 106.

With the emergence of next-generation reconfigurable optical add-dropmultiplexers (ROADMs) and all-optical wavelength selective switches(WSSs), carriers are starting to demand that the same level ofautomation and connection provisioning capabilities currently availablewith SONET/SDH, ATM, MPLS, etc. also be provided at the all-optical(photonic) layer.

The present invention enables the control plane 106 to includeadditional constraints related to the physical transmission propertiesof the fiber plant, amplifiers, transceivers, and other optical networkelements before determining the validity of a lightpath connectionthrough the network 100. For illustration purposes, the optical network100 shows only the nodes 102, but those of ordinary skill in the artwill recognize that intermediate amplifiers and the like are typicallyincluded. Advantageously, the additional constraints enable the network100 to determine if a new lightpath connection satisfies the dualconstraints of physical and logical viability.

Some path computation work has started within the IETF on the PathComputation Element (PCE) but this has primarily focused on the problemof routing connections across multiple separate Interior-NNI domains.

Referring to FIG. 2, an illustration of the different types of softwaremodules associated with an optical control plane 106 (FIG. 1) isillustrated according to an exemplary embodiment of the presentinvention. Central to this architecture is an online Path Computationfunction. This function implements a distributed path computationalgorithm and provides path selection and next hop resolution based oninformation contained in the topology database.

The transaction time to setup a connection using distributed controlplane signaling with distributed path computation is approximately equalto about one round trip time, plus the sum of “per-node” processingtimes. At each node, the dominant amount of processing time is due topath computation. When considering the physical limitations of opticalpropagation, this computation time can seriously degrade the time to setup or restore an optical lightpath.

By using offline pre-computation, the online path computation functioncan be replaced with a path “selection” function. This change isexpected to save 10s to 100s of milliseconds per transaction and is acritical enabler for very fast service establishment and fast servicerestoration. Pre-computed routes will be periodically recomputed toaccommodate changes in network resource availability, and to raise theprobability that a pre-computed path can be successfully establishedwhen needed.

Referring to FIG. 3, a hybrid path computation capability that includesa centralized (offline) network planning tool at the network managementlayer and distributed (online) path computation engines at each networkelement for use by the control plane 106 (FIG. 1) is illustratedaccording to an exemplary embodiment of the present invention. Theoffline pre-computation has the benefit of being able to optimize basedon a broad set of information, and being able to pre-compute pathsacross multiple network elements. The function of the centralizednetwork planning tool is to pre-compute the validity of lightpaths toeach destination from each location. Input parameters to the networkplanning tool are derived automatically where possible from the networkvia control plane discovery and routing. A database of valid, reachabledestinations from each source is pre-computed and then disseminated toeach node to be used in a distributed route calculation.

FIG. 3 illustrates an example flowchart view of an automated connectionsetup procedure within an all-optical signaling and routing framework.There are two stages in this procedure; (i) offline stage and (ii)online (runtime) stage.

The offline (or pre-processing) stage is separated from the networkelement and may be located within the network management sub-system, forexample. The network topology, its components' characteristics and thecurrent network state are used along with the physical models togenerate a decision engine. The decision engine is defined as thetime-varying construct that is used during the runtime processing stageto readily obtain the physical validity of the set of expected trafficconnections in the network.

The online (or runtime processing) stage is located within the controlplane sub-system with access to the distributed control plane routingprotocol. It uses the results of the decision engine generator incombination with routing and wavelength assignment algorithms and thecarrier's route preferences to determine the set of valid paths amongwhich it selects the best path to satisfy the traffic demand. A keyfactor in determining the physical validity is the use of the currentnetwork state by the decision engine.

There are multiple ways to implement the decision engine either as afinite state machine, rule-based database or as a time-varying set ofmatrix constructs. This offline procedure can be designed to runperiodically based on the changes in network traffic or it can betriggered by a change in the topology (such as the addition of a newlink) or change in network characteristics (such as an amplifiercontroller that increases the amplifier gain to increase the supportedset of wavelengths). Of course, the network characteristics can also bechanged dynamically based on the feedback from the runtime processingstage. Although the framework will not preclude such feedback, thesemechanisms are part of the physics realm and are beyond the scope ofthis document.

Referring to FIG. 4, a hybrid path computation mechanism is illustratedin an optical network according to an exemplary embodiment of thepresent invention.

The traffic engineering (TE) database associated with the centralized(offline) network planning tool receives all routing updates distributedby the control plane. Thus, the centralized TE database is a mirror ofthe distributed databases located on each network element and thereforeholds an accurate representation of the network state. Periodically, thecentralized (offline) network planning tool calculates the top candidateoptical paths from each node to each other node based on the latest‘snapshot’ of network resource usage. Once calculated, this routeinformation is disseminated to each of the distributed network elementsin the form of a lookup table for use by the distributed (online) pathcomputation (path selection) function. The online path computation thenbecomes a process of:

1. find least cost available path from source to destination from thelookup table; and

2. if there is no such path, look at 2 hop paths with regenerator inbetween.

Clearly, there are a number of issues that remain to be solved. Thefrequency of path calculation and information dissemination remains tobe determined, as does the number of candidate paths from each node toeach other node. The reach performance of each path will vary dependingon the characteristics of the optical source (such as bitrate, type ofFEC, etc). And, such issues will impact the size and scalability of thelookup table.

In an agile WDM network, the use of offline network design tools thatcombine both optical propagation physics and traffic routing algorithmswill be increasingly important for network planning. At higher (logical)network layers there is no need to worry about the physics of light whencalculating paths across a network. At the optical (physical) layer,however, it is necessary to include an understanding of opticalpropagation when determining link attributes and status for optimalconnectivity. Because of their interactive nature, physical propagationand optimization calculations are both processor and time intensive andare not suited to online real-time path calculations.

The offline path pre-computation function includes optimizationalgorithms and software implementations of those algorithms thatidentify optimal network element placement, perform offline service pathpre-computation, and allow for a global optimization of restorationpaths.

The main goal of the offline path pre-computation function is torecommend a set of validated optical paths to the distributed networkelements to as to maintain an optimized network design. Using accuratenetwork topology, traffic and resource status information derived fromthe control plane, optimized primary and restoration network paths canbe calculated that deliver the specified QoS performance. A keyrequirement is to ensure that the necessary path diversity exists tomeet the failure survivability criteria (e.g. single, double and triplesimultaneous failure support) imposed on different services.

Additionally, due to the addition of new services or changes to existingservice endpoints, a certain amount of traffic rerouting may bedesirable to decrease network inefficiency (stranded capacity) inducedby traffic churn. It is envisaged that re-optimization of networkresources is a semi-static function that will occur at predeterminedintervals or upon certain network thresholds being exceeded, and isbased on traffic growth forecasts and current network status.

Key network metrics for use by the control plane path computation engineneed to be defined to aid in its rapid set-up and restorationactivities.

It has been proposed that automated validation testing of the offlinesimulation results be performed to confirm or validate that proposedlightpaths are indeed acceptable. Such an approach could take advantageof automated connection testing between test sets during off peak hours(perhaps associated with maintenance windows) using test/referenceoptical equipment. The theoretically calculated reach informationgenerated by the offline design tool would then be validated againstreal network reach data and thus provide the carrier with confidence.During such validation testing, it is proposed that the carrier mark newcircuits in a testing or maintenance mode, where the connection would beunavailable for live traffic. After validation, new circuits would bemarked as available.

Referring to FIG. 5, a network management system is illustrated with apath pre-computation function according to an exemplary embodiment ofthe present invention. Service path pre-computation and globalrestoration planning relies on data extracted directly from networkelements to obtain an accurate and timely view of resource utilization.This task also develops requirements for the network element to networkmanagement system (NMS) interface to support service invocation andmodification, as well as the transfer of relevant network utilizationdata from optical network layer to the offline capacity optimizationapplication.

It is proposed that an optical path pre-computation function bedeveloped as an extension to an Optical Design Tool. The new PhotonicPath Computation Tool would interface to the network in the same manneras existing tools, such as via a Universal OSS Gateway. Thus, thepresent invention contemplates the feedback of real performancemeasurements (including coherent transponders) into the modeling tool,subsequent calculations of path viability and the distribution of apath-viability matrix in the NE's through a control plane.

Referring to FIG. 6, the optical network 100 is illustrated with aplurality of optical probes 600 included at trafficorigination/termination locations according to an exemplary embodimentof the present invention. The optical probes 600 are configured toobtain an accurate indication of the optical layer characteristics oneach of the fiber links 104 and to provide these characteristics to thecontrol plane 106. Each of the optical probes 600 includes a transmitterand a receiver. Further, the optical probes 600 can be positioned at alltraffic origination/termination points or a subset thereof. The opticalprobes 600 validate path computation functionality in the control plane106 by determining physical layer viability in addition to logical layerviability.

The optical probes 600 can include a Homodyne or an Intradyne Coherentreceiver, with a digital post-processor. The Coherent receiver providesa direct measure of incident electrical field (not power) amplitude andphase. As such, full information on the linear, nonlinear and noiseeffects is fully preserved. An Analog-Digital converter digitizes theincoming analog signal, such that a digital signal processor can beapplied to signal analysis. Homodyne implies that the carrier and thelocal oscillator (LO) are phase locked to each other. In Intradynecoherent receivers, the LO and carrier are not phase locked, but withina frequency offset which keeps the beat products of the informationspectrum within the electrical bandwidth of the receiver. For example,operation can be with a frequency difference of up to 500 MHz. Also,coherent transponders are dual-polarization allowing the exploration ofpolarization simultaneously through the use of the orthogonal signals.

Since electric field is directly measured, the optical probes 600provide a direct view into channel distortions. Analysis of DSP filtercoefficients indicates the level of Polarization Dependent Loss (PDL),Polarization Mode Dispersion (PMD), Chromatic Dispersion and FilterNarrowing effects. More sophisticated processing signal processing, suchas applying reverse Schrodinger propagation can be used to estimateintra-channel nonlinear effects, such as SPM.

Some impairments can be computed in real time on the incoming data,while others may require data storage and more sophisticatedcomputational post-processing.

The same Coherent receiver can be applied to any of the existingchannels, regardless of bit rate or modulation format, so long aschannel is within the receiver bandwidth (to prevent aliasing issues).The optical probe 600 provides a simple optical pulse source, forexample, selectable from 50 ps or 12 ps pulses. This probe can becoupled with an optical impairment monitor that is capable ofdifferentiating ASE, CD, PMD, and SPM accumulation. Advantageously, theprobe 600 is capable of a variety of measurements for different bitrates, modulation formats, etc. Note, typically, new transmitters comeout frequently, e.g. 10 G chirped/unchirped, 10 G with/without EDC, 40 Gduobinary/DPSK/DQPSK, 100 G, and the probe 600 is capable of providingmeasurements for all of these.

In order to probe newly commissioned paths that may not haverepresentative channels loaded, the optical probe 600 provides a reducedcomplexity transmitter. The transmitter generates internal data, and canprogrammably modify its transmission properties, such as data rate andsome format parameters. That is, the optical probe 600 includes atransmitter solely generating and transmitting first selectivelyvariable test data to another optical probe 600 and a receiver receivingand processing second selectively variable test data from the anotheroptical probe 600. In this manner, the optical probe 600 isdistinguishable from conventional monitor circuits in receivers or thelike that monitor in-service optical channels. The optical probe 600 isconfigured to measure unequipped light paths meaning there are noin-service channels at the wavelength over which the optical probe 600is operating. This enables dynamic optical measurements prior tobringing optical channels in-service such as with the control plane.

Referring to FIG. 7, a dynamic probe transmitter 700 for the opticalprobe 600 is illustrated according to an exemplary embodiment of thepresent invention. The dynamic probe transmitter 700 includes atransmitter 702, such as a tunable distributed feedback (DFB) laser,followed by a Mach-Zehnder modulator (MZM) 704. The modulator 704 canhave adjustable rate modulation input, and adjustable biasing 706, whichcan produce a rich variety of pulse train properties with alow-complexity hardware configuration. For example, the adjustablebiasing 706 can be tuned to Null, Quadrature, or Peak bias, producing67%, 50%, and 33% pulses respectfully, with either Carrier orCarrier-Suppressed configurations.

The modulator 704 may be driven by a simple oscillator 708, althoughthis reduces the frequency content of the generated signal. Theoscillator 708 can be rate selectable to provide various different bitrates, e.g. 10 G, 22 G, 28 G, 40 G, 100 G, etc. Additionally, thetransmitter 700 could use shift registers 710 or some other means togenerate a PRBS sequence, which may be of relatively short length. Thisprovides more spectrally dense signal frequency content, depending onthe PRBS sequence length.

The laser 702 can be tunable to provide access to all possiblewavelength slots in the system. Further, an optional polarizationrotator 712 may be added to provide access to additionalpolarization-dependent information, such as PDG, PDL and PMD, within amuch shorter time frame than would be available from natural systemfluctuations.

It should be noted that other configurations of the transmitter 700 arealso contemplated by the present invention. For example, a directlymodulated laser can be used to further reduce the cost and complexity,but at the expense of variety of selectable pulse properties.

Referring to FIG. 8, a dynamic probe receiver 800 for the optical probe600 is illustrated according to an exemplary embodiment of the presentinvention. The receiver 800 is implemented as a Coherent,Digitally-Sampled configuration through an A-to-D converter 802, withback-end DSP processing 804. The DSP 804 can include dynamicallyadaptive filters that may be synchronized with the specific data ratebeing transmitted by the probe.

A further improvement may be to include additional data path andelectronic circuitry to provide improved common-mode signal rejection.In this way, a coherent receiver can operate without an opticalpre-filter, and can be tuned to any channel in the system purely throughLO frequency selection, and subsequent DSP processing.

It should also be noted that the same Coherent receiver can be used toprobe existing data carrying channels, and may be used to estimate reallive traffic performance characteristics along the transmission path.Further, same arrangement may be used to provide eavesdroppingcapability. Here, the Coherent receiver can receive a split signal froman existing data carrying channel without use of the transmitter on theoptical probe 600.

Various mechanisms exist for estimating some optical propagationcharacteristics, such as residual Chromatic Dispersion, InstantaneousPMD impairment, and Optical Signal Noise Ratio. For example, somemechanisms are described in Hauske et al., “Optical PerformanceMonitoring from FIR Filter Coefficients in Coherent Receivers,” OFCNFOEC2008, Optical Society of America, 2008; Hauske et al., “DGD Estimationfrom FIR Filter Taps in Presence of Higher Order PMD,” ECOC 2008,September 2008; and Yi et al., “Experimental Demonstration of OpticalPerformance Monitoring in Coherent Optical OFDM Systems,” OFC/NFOEC2008, Optical Society of America, 2008, the contents of each are hereinincorporated by reference.

Another approach to estimate OSNR may be to use the fact that ASE noiseaccumulates on both polarizations, while the probe signal is transmittedon only one.

Optical filter (as for example, caused by in-line ROADMs) bandwidthnarrowing can be probed via investigating the dependence of pulse shapeon transmitted bit rate.

Other parameters, such as Self Phase Modulation may be estimated bylooking at the relationship of optical phase deviation from nominal as afunction of optical pulse power and optical pulse rate/duration. Amountof SPM is proportional to the derivative of power relative to time (i.e.higher-power and shorter pulses accumulate more SPM).

Another mechanism for estimating SPM could be to apply reverseSchrodinger propagation and optimize on the best received signal byestimating the unwrapping of generated nonlinear phase shift.

Cross-Phase modulation is more difficult to estimate. Though somepossibilities may include subtracting out polarization-dependent OSNRestimates, and looking at residual noise. For example, ASE noise dependson ‘Probe’ signal power, and would decrease as Probe power is increased.However, XPM noise is independent of Probe power, and depends only onnear neighbors. Thus, mapping out noise dependence on Probe channelpower can allow ASE and XPM separation.

The proposed invention provides an accurate estimation for opticalchannel performance characteristics, based either on existing channelsor on an additional dynamic optical probe. It provides an ability toobtain all critical parameters, such as:

1. More accurate OSNR measurement

2. Estimation for residual Chromatic Dispersion

3. Estimation for Polarization Dependent Loss

4. Estimation for Polarization Mode Dispersion

5. Estimation for inter-channel nonlinear effects, such as XPM and FWM

6. Estimation for intra-channel nonlinear effects, such as SPM, iXPM,iFWM

7. Estimation for possible bandwidth narrowing due to in-line OADMfilters

These advantages can be traded for a significant enhancement in theperformance and efficiency of deployed optical networks. Improvednetwork performance can be translated into a combination of higheroperational reliability and lower capital costs.

1. Network bandwidth can be allocated in a more efficient manner, withtraffic routed over links with inherently better performance and highercapacity

2. Total link capacity can be fine-tuned on the installed system torecover inherent margin associated with deployment uncertainties andaging.

3. Individual channels can be optimized, accounting for actual deployedhardware characteristics.

As DSPs, ADCs and DACs become smaller, more power efficient and lessexpensive, they can be used for lower bit-rate transmission to maketransponders much simpler to yield and higher performance. This samething has happened in radio technology, i.e. no one would consider adirect detection radio even for the simplest, most cost challenged ofapplications. Also, it is the expectation that low cost coherent systemswill not be limited by digital bandwidth such that one can consider muchmore complex schemes in the future at essentially the same cost. Forexample, one can consider using heterodyne detection. If one is notchallenged by the digital performance, then one could consideroversampling the heterodyne intermediate frequency (IF) and then provideadaptive digital filtering. For instance, in an optical service channel(OSC) application, 1.55 Mbps can be sent through a transponder with a 5Gsample/s capability and a matching 5 GHz electrical bandwidth. Thedigital system could then do the filtering down to the ˜1.5 MHz receiverbandwidth thereby retaining tunability, noise performance andsensitivity. The limit in this case would be the common mode rejectionratio (CMRR) which can be quite good. Today, one can easily achieve 50dB with parts from a specific vendor (although the CMRR degrades withincreasing IF which could limit the offset to <5 GHz—though this couldbe improved in the future and potentially even calibrated overfrequency, self-calibration also is a possibility). One could also beable to tune and track the carrier to explore the phase response of thechannel—indeed the possibilities are endless.

Referring to FIG. 9, a block diagram illustrates a server 900 configuredto, responsive to computer-executable code, perform an optical pathcomputation function according to an exemplary embodiment of the presentinvention. The server 900 can be a digital computer that, in terms ofhardware architecture, generally includes a processor 902, input/output(I/O) interfaces 904, network interfaces 906, a data store 908, andmemory 910. The components (902, 904, 906, 908, and 910) arecommunicatively coupled via a local interface 912. The local interface912 can be, for example but not limited to, one or more buses or otherwired or wireless connections, as is known in the art. The localinterface 912 can have additional elements, which are omitted forsimplicity, such as controllers, buffers (caches), drivers, repeaters,and receivers, among many others, to enable communications. Further, thelocal interface 912 can include address, control, and/or dataconnections to enable appropriate communications among theaforementioned components.

The processor 902 is a hardware device for executing softwareinstructions. The processor 902 can be any custom made or commerciallyavailable processor, a central processing unit (CPU), an auxiliaryprocessor among several processors associated with the server 900, asemiconductor-based microprocessor (in the form of a microchip or chipset), or generally any device for executing software instructions. Whenthe server 900 is in operation, the processor 902 is configured toexecute software stored within the memory 910, to communicate data toand from the memory 910, and to generally control operations of theserver 900 pursuant to the software instructions.

The I/O interfaces 904 can be used to receive user input from and/or forproviding system output to one or more devices or components. User inputcan be provided via, for example, a keyboard and/or a mouse. Systemoutput can be provided via a display device and a printer (not shown).I/O interfaces 904 can include, for example, a serial port, a parallelport, a small computer system interface (SCSI), an infrared (IR)interface, a radio frequency (RF) interface, and/or a universal serialbus (USB) interface.

The network interfaces 906 can be used to enable the server 900 tocommunicate on a network. The network interfaces 906 can include, forexample, an Ethernet card (e.g., 10BaseT, Fast Ethernet, GigabitEthernet) or a wireless local area network (WLAN) card (e.g.,802.11a/b/g/n). The network interfaces 906 can include address, control,and/or data connections to enable appropriate communications on thenetwork. A user can log on and communicate with the server 900 remotelythrough the network interfaces 906. In the present invention, thenetwork interfaces 906 can be configured to communicate to various nodesand optical probes to retrieve physical layer attributes used in a pathcomputation.

A data store 908 can be used to store data, such as fitness data, MDdata, etc. The data store 908 can include any of volatile memoryelements (e.g., random access memory (RAM, such as DRAM, SRAM, SDRAM,and the like)), nonvolatile memory elements (e.g., ROM, hard drive,tape, CDROM, and the like), and combinations thereof Moreover, the datastore 908 can incorporate electronic, magnetic, optical, and/or othertypes of storage media. In one example, the data store 908 can belocated internal to the server 900 such as, for example, an internalhard drive connected to the local interface 912 in the server 900.

The memory 910 can include any of volatile memory elements (e.g., randomaccess memory (RAM, such as DRAM, SRAM, SDRAM, etc.)), nonvolatilememory elements (e.g., ROM, hard drive, tape, CDROM, etc.), andcombinations thereof Moreover, the memory 910 may incorporateelectronic, magnetic, optical, and/or other types of storage media. Notethat the memory 910 can have a distributed architecture, where variouscomponents are situated remotely from one another, but can be accessedby the processor 902.

The software in memory 910 can include one or more software programs,each of which includes an ordered listing of executable instructions forimplementing logical functions. In the example of FIG. 9, the softwarein the memory 910 includes a suitable operating system (O/S) 920 and apath computation 922 program. The operating system 920 essentiallycontrols the execution of other computer programs, such as the pathcomputation 922 program, and provides scheduling, input-output control,file and data management, memory management, and communication controland related services. The operating system 920 can be any of Windows NT,Windows 2000, Windows XP, Windows Vista (all available from Microsoft,Corp. of Redmond, Wash.), Solaris (available from Sun Microsystems, Inc.of Palo Alto, Calif.), LINUX (or another UNIX variant) (available fromRed Hat of Raleigh, N.C.), or the like.

Referring to FIGS. 10 and 11, a coherent OSC system 1000 is illustratedaccording to an exemplary embodiment of the present invention. Thecoherent OSC system 1000 is specifically designed for a low bit ratecommensurate with OSC applications, and employs a modulation formatwhich is very robust, for example 1 Gbps dual-polarization binaryphase-shift keying (DP-BPSK). FIG. 10 is a functional schematic diagramshowing various components of the coherent OSC system 1000 such as anOSC Tx 1002, an OSC Rx 1004, four-port couplers 1006, and an optionalvariable optical attenuator (VOA) or shutter 1008. The four-portcouplers 1006 are illustrated in additional detail in FIG. 11. Thecoherent OSC system 1000 can be package and a module with the variouscomponents 1002, 1004, 1006, 1008 contained therein. The OSC Tx 1002 canemploy a fixed laser, or tunable where tunability enables more features.The coherent OSC system 1000 can also include a DSP capable of measuringvarious parameters, many of which are statistical in nature, whichcharacterize the channel over which the OSC has propagated, similar tothe optical probe 600. This DSP may be a part of the DSP which enablesthe coherent detection, or it may be a separate block. The coherent OSCsystem 1000 is used in an optical network to automatically detectPropagation Distance, PMD, PDL, CD, OSNR, Rx Power, and the like similarto the optical probe 600. With a tunable laser variant, this can beswept across available wavelength positions.

The four-port coupler 1006 coupled to the OSC Tx 1002 is configured toinject the OSC wavelength onto the transmission fiber and the four-portcoupler 1006 at the end of the transmission fiber coupled to the OSC Rx1004 removes a portion of the light travelling on the fiber and directsit to the OSC Rx 1004 for processing. For example, the coherent OSCsystem 1000 can be duplicated and used with a pair of transmissionfibers arranged to provide a bidirectional link for communicationsbetween nodes. It is possible for the transmission fibers to carry twodirections of data-bearing traffic channels and for the OSC channel totravel in the same direction (co-propagating) as these channels, or inthe opposite direction (counter-propagating) from the data channels.Another exemplary embodiment could use a single fiber working solution,where two different wavelengths are used by the OSC to provide abidirectional link on the single working fiber. Additionally, the OSCsystem 1000 can include an amplifier 1010 between the four-port couplers1006 and the OSC Rx 1004 and the OSC Tx 1002.

The four-port coupler 1006 can include four ports 1021, 1022, 1023,1024. The port 1021 is a common in port, the port 1022 is a common outport, the port 1023 is an OSC add port and the port 1024 is an OSCbypass port. For example, assume the OSC wavelength is 1510 nm (note:any wavelength is contemplated herein), the four-port coupler 1006 canbe a dielectric filter where 1510 nm is passed from the port 1023 to theport 1022 and from the port 1021 to the port 1024. The OSC Tx 1002contemplate a tunable wavelength such as 1500 to 1600 nm or the like.When the OSC wavelength is tuned to 1510 nm, it is transmitted by thefour-port coupler 1006 to the common out port 1022, and when it is tunedto any other wavelength than 1510 nm, it is reflected to the OSC bypassport 1024. In this manner, the OSC system 1000 can be used as both anOSC (e.g., at 1510 nm) and as a probe (e.g., at any traffic carryingwavelength). The inherent isolation of the dielectric filter (e.g., ˜70dB) is exploited to prevent multi-phase interference (MPI) for theamplification band of an erbium doped fiber amplifier (EDFA) such as theamplifier 1010.

Also, for the OSC wavelength at 1510 nm, interference is avoided sincethis wavelength is outside the amplification band of an EDFA. The inputport 1023 can include the VOA or shutter 1008 to allow the OSC Rx 1004to see only the OSC wavelength (e.g., 1510 nm) when high sensitivity isrequired. When operating in-band, the OSC Tx 1002 is bypassed to theinput of the amplifier 1010 where it experiences gain. The downstreamOSC Rx 1004 then sees the in-band OSC wavelength from any upstreaminsertion point (note, only one such insertion point at a givenwavelength is allowable since it will propagate downstream). The opticalsignal from the in-band OSC wavelength will include ASE and the likefrom all upstream amplifiers, allowing measurements of the cumulativenoise and other parameters of interest on the path.

Similar to the optical probe 600, using a coherent OSC with a highcommon-mode rejection ratio (CMRR) for the OSC Tx 1002, it is possibleto inject this channel into an unused portion of the transmission bandwhere it can propagate in the same part of the spectrum as the databearing channels. Therefore, the four-port coupler 1006 to thetransmission fiber can include a broad-band tap, a specific wavelengthfilter (either in-band or out-of-band), or a tunable filter which may beattached to a tap or of a 3-port variety, for example a wavelengthselective switch. Accordingly, the OSC system 1000 could have a featurecalled Probe Mode. In Probe Mode, the OSC coherent Rx 1004 is used tomeasure the statistics of the phase noise. Using a tunable transponderfor the OSC Tx 1002 and the four-port coupler 1006 which allows theadjustment of the wavelength in use, the OSC system 1000 can be adjustedto a portion of the spectrum which is unused and which propagatesthrough the amplifiers 1010. This can happen in service if it iscoordinated between sites giving the ability for multiple OSC Rx 1004 todetect the same signal as it propagates through the line. The injectionpoint for this channel can also be coordinated. Phase noise statisticscan be extracted and accurate conclusions can be made on XPM and OSNR.By adjusting with powers, understanding OSNR for a given launch power,and perhaps applying special modulation on the probe tone, SPM can beestimated. In normal operation, the OSC system 1000 can be adjusted toan out-of-band wavelength carrying data between sites. Also, the servicechannel could be supported on very challenging optical networks, forexample ultra long haul systems and submarine systems, since the OSCsystem 1000 includes a robust modulation format and low bit-rate.

Advantageously, with a coherent service channel in a network, it can beused to auto-detect link budgets and suggest the correct modem forsupport (i.e. no simulations required—real plug and play). It canestimate maximum capacity. It can also be used to monitor lineconditions, and historical latency and distance performance monitoring(PMs) data can be added to the service channel capabilities (i.e. nomore field trials and test sets for latency measurements). Since the OSCsystem 1000 works on a span-by-span basis, it allows location of theseeffects to the specific fiber of interest. In another exemplaryembodiment, an application to Submarine links, or on 3rd party lines,the OSC system 1000 can takes the form of a card which can be injectedand removed for detection on a link-by-link basis. In theseapplications, the OSC system 1000 and the probe 600 allows themeasurement of information and location of potential issues directlythrough the line system even with the lack of detailed knowledge of thedesign and operation of the system in question.

Some portions of the aforementioned detailed descriptions are presentedin terms of procedures, logic blocks, processing, steps, and othersymbolic representations of operations on data bits within a computermemory. These descriptions and representations are the means used bythose skilled in the data processing arts to most effectively convey thesubstance of their work to others skilled in the art. A procedure, logicblock, process, etc., is generally conceived to be a self-consistentsequence of steps or instructions leading to a desired result. The stepsrequire physical manipulations of physical quantities. Usually, thoughnot necessarily, these quantities take the form of electrical ormagnetic signals capable of being stored, transferred, combined,compared and otherwise manipulated in a computer system. It has provenconvenient at times, principally for reasons of common usage, to referto these signals as bits, bytes, words, values, elements, symbols,characters, terms, numbers, or the like.

It should be born in mind that all of the above and similar terms are tobe associated with the appropriate physical quantities they representand are merely convenient labels applied to these quantities. Unlessspecifically stated otherwise as apparent from the followingdiscussions, it is appreciated that throughout the present invention,discussions utilizing terms such as ‘processing,’ ‘computing,’‘calculating,’ ‘determining,’ ‘displaying’ or the like, refer to theaction and processes of a computer system, or similar electroniccomputing device, that manipulates and transforms data represented asphysical (electronic) quantities within the computer system's registersand memories into other data similarly represented as physicalquantities within the computer system memories or registers or othersuch information storage, transmission or display devices.

The invention can take the form of an entirely hardware embodiment, anentirely software embodiment or an embodiment containing a combinationof hardware and software elements. In one embodiment, a portion of themechanism of the invention is implemented in software, which includesbut is not limited to firmware, resident software, object code, assemblycode, microcode, etc.

Furthermore, the invention can take the form of a computer programproduct accessible from a computer-usable or computer-readable mediumproviding program code for use by or in connection with a computer orany instruction execution system. For the purposes of this description,a computer-usable or computer readable medium is any apparatus that cancontain, store, communicate, propagate, or transport the program for useby or in connection with the instruction execution system, apparatus, ordevice, e.g., floppy disks, removable hard drives, computer filescomprising source code or object code, flash semiconductor memory (USBflash drives, etc.), ROM, EPROM, or other semiconductor memory devices.

Although the present invention has been illustrated and described hereinwith reference to preferred embodiments and specific examples thereof,it will be readily apparent to those of ordinary skill in the art thatother embodiments and examples may perform similar functions and/orachieve like results. All such equivalent embodiments and examples arewithin the spirit and scope of the present invention, are contemplatedthereby, and are intended to be covered by the following claims.

What is claimed is:
 1. A method for path computation based on dynamicperformance monitoring in an optical network, the method comprising:obtaining dynamic performance monitoring data related to optical layercharacteristics in the optical network; performing an offlinecomputation to generate a decision engine for determining physicalvalidity of paths in the optical network based on the dynamicperformance monitoring data, wherein the decision engine provides validand reachable destinations from each source network element; anddisseminating the decision engine to network elements in the opticalnetwork for use in path computation at runtime.
 2. The method of claim1, further comprising: periodically repeating the obtaining, theperforming, and the disseminating.
 3. The method of claim 1, wherein thedecision engine is utilized in the path computation by a network elementcomputing a path at runtime to determine physical validity of the pathin addition to logical validity of the path.
 4. The method of claim 1,wherein the decision engine is one of a finite state machine, arule-based database, and as a time-varying set of matrix constructs. 5.The method of claim 1, further comprising: computing offline a pluralityof top candidate optical paths from each node to each other node basedon a current snapshot of network resource usage; and disseminating theplurality of top candidate paths to each of the network elements.
 6. Themethod of claim 1, wherein the dynamic performance monitoring datacomprises feedback from real optical performance measurements in theoptical network, and wherein the offline computation utilizes the realoptical performance measurements in modeling and calculations to ensureoptical path viability.
 7. The method of claim 6, wherein the realoptical performance measurements are obtained from any of coherenttransceivers, coherent probes, and coherent Optical Service Channels(OSCs).
 8. The method of claim 1, wherein the obtaining, the performing,and the disseminating is performed by a centralized server and thenetwork elements operate a control plane with an individual networkelement utilizing the decision engine in path computation via thecontrol plane.
 9. A server adapted to perform path computation based ondynamic performance monitoring in an optical network, the servercomprising: a network interface and a processor communicatively coupledto one another; and memory storing instructions that, when executed,cause the processor to obtain via the network interface dynamicperformance monitoring data related to optical layer characteristics inthe optical network, perform an offline computation to generate adecision engine for determining physical validity of paths in theoptical network based on the dynamic performance monitoring data,wherein the decision engine provides valid and reachable destinationsfrom each source network element, and disseminate via the networkinterface the decision engine to network elements in the optical networkfor use in path computation at runtime.
 10. The server of claim 9,wherein the memory storing instructions that, when executed, furthercause the processor to periodically repeat the obtain, the perform, andthe disseminate.
 11. The server of claim 9, wherein the decision engineis utilized in the path computation by a network element computing apath at runtime to determine physical validity of the path in additionto logical validity of the path.
 12. The server of claim 9, wherein thedecision engine is one of a finite state machine, a rule-based database,and as a time-varying set of matrix constructs.
 13. The server of claim9, wherein the memory storing instructions that, when executed, furthercause the processor to compute offline a plurality of top candidateoptical paths from each node to each other node based on a currentsnapshot of network resource usage; and disseminate the plurality of topcandidate paths to each of the network elements.
 14. The server of claim9, wherein the dynamic performance monitoring data comprises feedbackfrom real optical performance measurements in the optical network, andwherein the offline computation utilizes the real optical performancemeasurements in modeling and calculations to ensure optical pathviability.
 15. The server of claim 14, wherein the real opticalperformance measurements are obtained from any of coherent transceivers,coherent probes, and coherent Optical Service Channels (OSCs).
 16. Theserver of claim 9, wherein the network elements operate a control planewith an individual network element utilizing the decision engine in pathcomputation via the control plane.
 17. Software stored in one ofvolatile and non-volatile memory comprising instructions that, whenexecuted, cause a processor to perform steps of: obtaining dynamicperformance monitoring data related to optical layer characteristics inthe optical network; performing an offline computation to generate adecision engine for determining physical validity of paths in theoptical network based on the dynamic performance monitoring data,wherein the decision engine provides valid and reachable destinationsfrom each source network element; and disseminating the decision engineto network elements in the optical network for use in path computationat runtime.
 18. The software stored in one of volatile and non-volatilememory of claim 17, wherein the instructions that, when executed,further cause a processor to perform steps of: periodically repeatingthe obtaining, the performing, and the disseminating.
 19. The softwarestored in one of volatile and non-volatile memory of claim 17, whereinthe decision engine is utilized in the path computation by a networkelement computing a path at runtime to determine physical validity ofthe path in addition to logical validity of the path.
 20. The softwarestored in one of volatile and non-volatile memory of claim 17, whereinthe decision engine is one of a finite state machine, a rule-baseddatabase, and as a time-varying set of matrix constructs.